The invention relates to an arrangement for the input of energy into a gas-swept electrical discharge, particularly a gas laser, comprising a discharge chamber through which a gas is flowing at a high velocity and in which the gas discharge burns as a steady-field discharge between an anode and at least one cathode opposite the anode. A controllable switched-mode power supply is used to supply power to the electrodes.
An arrangement of this kind is known from DE-OS 34 46 145. This publication describes a direct current discharge gas laser having a power supply unit which is provided with a controllable alternating current source with a high internal impedance, a rectifier disposed downstream thereof and a charging capacitor which is connected to the output of the rectifier and parallel to the path of the gas discharge of the gas laser A transformer is interposed between the alternating current source and the charging capacitor for the purpose of galvanic separation. Further, it is possible to connect the primary windings of several transformers between the output terminals of the alternating current source with each of the secondary windings of the transformers each of which feeds one gas laser.
The total electric power required for the laser is supplied via the semiconductor switching device and the galvanically separating intermediate transformers. The efficiency is high and amounts to approximately 85%.
This power transmission is suitable for longitudinal-flow lasers of up to a few kW and is frequently configured as such. For transverse-flow high-performance lasers of 5 kW and more, requiring an electrical power of 50-150 kW, the labor and costs involved in high-performance semiconductor switching devices and transformers suitable for an operation frequency of 20-300 kHz is unrealistically high so that solutions of this kind are not known.
Another discharge device in the form of a gas laser is known from U.S. Pat. No. 4,449,220. The electrode arrangement thereof includes a plate-like anode and a row of individual cathodes in the form of massive rods running parallel thereto and disposed close to a wall of the gas flow channel. The walls of the gas flow channel run parallel to one another, and the channel is formed by an anode elongated in flow direction. The problem of this arrangement, however, is that the individual cathodes are in the wake or backwash of each preceding individual cathode, except for the one disposed first in flow direction. These highly heated backwash areas favor instabilities and greatly impair the heat removal at these individual cathodes. A consequence thereof is the heavy oxidization of the rods particularly in case of high power densities which are usually required. In molybdenum or tungsten rods, this effect is encountered increasingly, beginning with 600.degree. C., for example. DE 28 56 328 to which U.S. Pat. No. 4,449,220 corresponds, further describes a steady-field pre-ionization which is generated by pin-like pre-ionization electrodes protruding from the wall of the discharge chamber. The auxiliary discharge in the area of this pre-ionization electrodes requires a power in the magnitude of 40% of the energy input.
To prevent excessive heating of the individual cathodes, use is often made of tubular cathodes, of copper for example, through which a coolant is pumped. Such an arrangement is disclosed in U.S. Pat. No. 4,077,018 wherein a gas laser is described in which the discharge chamber is surrounded by 3 individual electrodes through which a coolant constantly flows to remove the heat and give the cathodes a sufficiently long useful life. Experience shows, however, that the cathode life that can be achieved in this manner is sufficient to satisfy industrial requirements. The potential difference between the cathodes is invariably selected via ballast resistors.
Another discharge device in the form of a gas laser is known from U.S. Pat. No. 4,488,309. This gas laser has an anode which is expanded in flow direction and defines one side of the discharge channel for the gas. Several pin-like individual anodes running transversely to the gas flow and connected to the DC supply via a high-value starting resistor are located opposite this anode. A rod-like pre-ionization electrode having a conductive core enveloped by a dielectric coating is disposed centrally between the anode and the individual cathodes. The pre-ionization electrode is connected to an AC supply. Moreover, between the anode and the ground potential there is a capacitor and between the output of the power supply and the anode, there is in inductor which serves as an energy storage for pulse operation. Capacitor and inductor must be designed for a high pulse energy in order to maintain the continuous power of the DC supply at a low level. This arrangement permits generating a stable DC-glow discharge which can be easily switched and pulsed, i.e. it exhibits good starting properties; moreover, the discharge voltage of the DC-discharge decreases. In this arrangement, the pre-ionization discharge burns exclusively between the pre-ionization electrode and the anode since there are high-value starting resistors in the cathode circuit. A consequence thereof is that the gas volume in the discharge chamber is not uniformly pre-ionized and the relative power consumption is excessively high. The costs for the power supply are high since high-frequency alternating current is unreasonably expensive as compared to direct current.
An embodiment of a different kind of pre-ionization devices is known from WO 92/01281, to which U.S. Pat. No. 4,342,115 corresponds. This publication describes in particular the form of individual electrodes for steady-field pre-ionization. The possible forms mentioned include disks, T-like hooks, U-like hooks and L-like hooks which extend into the discharge chamber. This pre-ionization device requires a great amount of energy.
The increasing use of high-power lasers in material processing entails also increasingly high demands on economical and technical properties of the devices. Particular interest is attached to the overall efficiency of the laser and the regulated, fast control of the laser performance in order to select the desired mode of operation (for example cutting or welding) in a time range of &lt;1 ms. In addition to optimizing the efficiency of the individual components, the input of electrical power into the laser medium and the adjustment of the discharge volume to the resonator geometry (or vice-versa) are of utmost importance. Particularly in transverse-flow lasers, the process and the device of the invention permit a pulsable start of the laser medium of a particularly high efficiency in the desired frequency range.
Several different arrangements, which, however, do not simultaneously meet the above requirements, are known for electrically activating the laser-active medium of high-power lasers, particularly CO.sub.2 -lasers.
A DC-activation requires the starting resistors necessary for stabilizing the discharge which in turn decrease the efficiency to 60-70%. Moreover, in transverse-flow lasers, the electrodes require a frequent segmentation which together with the cooling necessary for high-power performances involves high construction costs.
The laser power is controlled by controlling the supply voltages by means of 6- or 12-pulse thyristor controllers or thyristor bridges. In order to reduce the voltage modulation (up to 100%) given at the phase interface control to a value of &lt;1% required for technical applications, it is necessary to use smoothing filters. These inductors and the relatively low system frequency of 50 Hz, which brings the bridge frequency to 300 Hz at the 6-pulse thyristor bridge and to 600 Hz at the 12-pulse thyristor bridge, extend the control delay to approximately 100 ms. This time constant is unacceptably long for many cutting and welding tasks.
The use of electron tubes as longitudinal controllers does provide a sufficient control speed, the overall efficiency, however, is reduced by theses tubes to such an extent that this technical option is only used in low-power longitudinal-flow lasers.
Further, it is known to use high-frequency generators which can be modulated in lasers in order to activate molecular gases. Electrical energy is input into the medium via dielectrically enveloped or purely metallic electrodes.
In high-frequency generators equipped with tubes for high-power performance and in circuits for compensating the blind component of the electrode system, the electrical overall efficiency ranges between 40 and 50%. All of the power required by the laser in the discharge chamber is supplied by the HF-generator.
The supply of actuating power from a modulable high-frequency generator, however, permits a regulated control of the laser energy to change the operational processes like cutting, welding and hardening at a time range of approximately 100 .mu.s. Since the system permitted a fast control, the high operating costs caused by the low efficiency of the HF-generator, the high maintenance costs and the high purchase costs had to be accepted.
The discharge geometry can be well matched with the geometry of the laser resonator only in case of longitudinal-flow lasers. When applying the transverse-flow laser principle which is preferred in a performance range of &gt;5 kW, this matching and hence a high optical efficiency of the resonator is little satisfactory.